The present invention relates to a method of optically monitoring a body having a reflective surface and a covert optical discontinuity produced on the surface.
It is often desirable to apply a covert mark to an object, for example, in order to identify goods or the like, which can be useful in anti-counterfeiting measures or which can allow for the application of a code specific to a container, thus facilitating product tracking.
Many types of covert marking systems have been proposed. It is known for example to mark objects using laser radiation. U.S. Pat. No. 4,758,703 describes a method of covertly encoding a microscopically visible pattern on a surface of an object in which a beam of unfocussed laser radiation is passed through a mesh to produce the desired pattern, the intensity of the laser beam being carefully controlled so that the pattern is barely etched onto the surface and remains visible to the eye. U.S. Pat. No. 4,769,310 describes a method of marking ceramic materials, glazes, glass ceramics and glasses that contain at least one radiation-sensitive additive in which a laser beam is focussed on to the surface of the material to be marked so as to induce a colour change within the irradiated area.
GB2247677 discloses a method and apparatus for providing a body of material with a sub-surface mark in the form of an area of increased opacity to electromagnetic radiation. The method comprises directing a high energy density beam to which the material is transparent and bringing the beam to focus at a location spaced from the surface and within the body so as to cause localized ionisation of the material. The apparatus includes a laser and provides means to move the focus of the beam relative to the body so as to enable the mark to be of a predetermined shape.
Such covert marking systems are generally only effective for glass or ceramic based products.